The invention relates to causing scale formation. In another aspect, the invention relates to causing selective scale formation along a portion of a path of a fluid stream.
In yet another aspect, the invention relates to causing scale formation in a screw expander utilizing a geothermally heated fluid as a power source.
Power can be generated from geothermally heated water and steam by utilizing, for example, plural rotor helical screw expanders such as disclosed in U.S. Pat. Nos. 3,751,673 and 3,977,818. Such devices operate well with geothermal sources which deliver a major portion of their energy as superheated water, much of which is hot brine laden with dissolved minerals. The dissolved mineral concentration of geothermal brines is high because of their in situ elevated temperature and pressure and tends to be at or near chemical saturation because of the long residence of the water in the mineral deposit. In the utilization of geothermal energy, the dissolved minerals in the product brines result in considerable problems associated with scale formation in all utilization equipment associated therewith.
When the brines are released from their geothermal source, the pressure and temperature are reduced and frequently there is an immediate precipitation of minerals. The amount of precipitate is frequently so great as to clog up and disable such utilization systems in a very short period, unless preventive measures are taken.
Helical screw expanders, in operation, continuously rid themselves of deleterious precipitated minerals. The minerals which do remain actually improve the efficiency of the expander by forming a seal between its moving surfaces, as well as reducing the possibility of corrosion attacking the internal expander surfaces. It is thus desirable to have scale deposits in helical screw expanders.
However, geothermal well fluids undergo a reduction in temperature and pressure as they are withdrawn from the well. The spontaneous deposition of minerals which can occur in the downhole piping string can completely clog up communication between the geothermal source and the screw expander in a short period. In order to bring the downhole piping string back on line, it is necessary to remove the offending deposit, at considerable expense and loss of time. To overcome the problems of downhole scaling, scale inhibiting compounds are injected at the bottom of the piping string. By regulating the amount of scale inhibiting compound admixed with the upwardly flowing fluid, it is possible to substantially prevent scale deposition in the downhole string. Scale deposition between the wellhead and the expander is normally not a problem, because the well fluids are at a substantially constant temperature and pressure along this portion of their path length.
A problem in utilizing well fluid which contains scale inhibiting compositions, or well fluids not containing high concentrations of dissolved minerals, is that deposition of minerals between the mating surfaces of the screw expander does not occur. It would thus be desirable when utilizing such geothermal sources as the motive fluid for an engine to provide for selective scale deposition in the engine to improve its efficiency.